In medical ultrasound imaging, it is often desired to perform an ultrasound scan at a selected time interval. Typically, the scan is synchronized to a selected event that is referenced to a repeating signal on a reference signal. For example, a selected event may be 100 ms in advance of an R-wave on an ECG signal.
It is also often desired to perform two or more scans at different but related times. For example, an operator can transmit one set of ultrasonic beams to destroy or activate contrast agent in a region at a specific time before transmitting a set of ultrasonic beams to image the region. It has recently been recognized that there are benefits to interrupting scanning to either preserve contrast in the scan plane and/or to provide time for fresh contrast agent to flow into the scan plane before the next scan. Some of the benefits of suspending ultrasonic transmissions are described in U.S. Pat. No. 5,560,364 (Porter) and include improved sensitivity to contrast agents. In addition to performing two scans at different but related times, an operator may desire to trigger imaging after a specific amount of time has elapsed since the activation of a surgical tool such as a laser or an infusion pump.
When two or more events need to be synchronized, they are typically all synchronized directly to a repeating signal on a reference signal, such as an R-wave on an ECG signal. Because these repeating signals are unsteady and susceptible to slow or rapid drifts in period and to transient aberrant behavior, these synchronizing techniques provide inaccurate means for triggering the additional scans in many ultrasound applications.